Modular electrical energy storage device

ABSTRACT

A compact modular electrical energy storage device comprising one or more cell modules in which certain components of the device are maintained in a substantially fixed position in the device by utilizing a resilient porous body of compressed carbon fibers to resiliently urge the components into physical contact with one another. The invention further provides means for maintaining the cell components in such fixed position during cell operation at elevated temperatures for extended periods of time. 
     A preferred compact cell module construction particularly suitable for use in a high-temperature, high-energy-density lithium battery includes two positive electrode assemblies positioned in opposing relationship by a U-shaped spacer member maintained in contact with a separator member of each positive electrode assembly by a resilient porous body of compressed graphite fibers, a unitary double-faced negative electrode assembly being positioned between the two positive electrode assemblies.

BACKGROUND OF INVENTION

1. Field of the Invention

The present invention relates to a rechargeable electrical energystorage device, and, more particularly, to a compact modularrechargeable cell construction which utilizes a molten salt electrolyte.

2. Prior Art

The problem of air pollution in urban areas attributed to emissions frommotor vehicles using internal combustion engines is of increasingconcern. Because battery-powered vehicles themselves produce no exhaustor unburned fuel emission, they are particularly attractive for urbanuse. However, to develop practical electrical automobiles for generaluse, low-cost secondary batteries having sufficient high-energy densityand high-power density are required. Liquid lithium metal has beenextensively utilized in some high-power density molten salt batteries,e.g., Li/Cl₂, Li/S, Li/Se, and Li/Te.

The lithium-sulfur cell using molten halide electrolytes is ofparticular interest. See M. L. Kyle et al., "Lithium/Sulfur Batteriesfor Electric Vehicle Propulsion", 1971 Sixth Intersociety EnergyConversion Engineering Conference Proceedings, p 38; L. A. Heredy etal., Proc. Intern, Electric Vehicle Symp., Electric Vehicle Council 1,375 (1969). However, it has been found that high self-discharge ratesdue to corrosion of cell components by liquid lithium coupled with someappreciable solubility of liquid lithium in the molten salt electrolytesoften cause difficulties in material selection and battery cell design.Such difficulties can be avoided by use of a solid alloy of lithium as asource of lithium in an electrochemical cell. One such alloy is thealuminum-lithium alloy which has been utilized as the solid negativeelectrode. Excellent electrochemical performance of aluminum-lithiumalloy in a composition range of 5-30 wt.% lithium in a molten saltelectrolyte has been reported. See N. P. Yao et al., "Emf Measurementsof Electrochemically Prepared Lithium-Aluminum Alloy", J. Electrochem,Soc. 118, 1039-1042 (July 1971) and references cited therein.

Heretofore, the principal reported effort toward the development of ahigh-energy-density battery has been directed toward improvement of theindividual active components of the cell or battery, namely, theelectrodes and electrolytes. There exists, however, another problem inthe development of such a battery, namely, a battery which utilizes amolten salt as the electrolyte is subject to sustained high-temperatureoperation as well as to considerable variation in temperature.Specifically, the battery temperature may range from ambient up to itsoperating temperature which normally is in excess of 200°C, generallyfrom about 350° to 450°C. The individual components of the battery havegreatly different thermal coefficients of expansion. Obviously, somemeans must be provided in the battery for maintaining the individualcomponents in a substantially fixed relationship with one another atthese elevated temperatures and over the given temperature range.

It has been proposed that the components be fixed in a certain spatialrelationship by utilizing a non-resilient means such as ceramic spacermembers and pins to key the components together. However, such atechnique does not allow for any movement of the components upon heatingof the battery to its operating temperature. With a fixed restrainingmeans, breakage of the individual components as a result of thermalexpansion is not uncommon. Obviously, there still is a need for anelectrical energy storage device wherein the individual components areretained in a desired relationship to one another, but which will stillprovide for some movement of the components as a result of thermalexpansion.

Furthermore, in order to retain the active material in the positiveelectrode assembly, a porous separator is used. A relatively bulkysupporting and retaining structure is then needed to hold the separatorin place. To the extent that the available space in the cell structureis not utilized by active material, a lower energy density results. Thusin order to obtain a high-energy-density battery composed of a pluralityof cells, which will be of practical interest for automotiveapplications and the like, the need exists for providing cell moduleshaving maximum utilization of the cell volume, while at the same timeproviding ready access to cell components, ease of assembly, andreliability over sustained periods of operation. The devices heretoforeproposed have been generally found lacking in several of theserequirements.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a compact modularelectric energy storage device, comprised of one or more cell modules,which overcomes the known disadvantages of prior art devices and moreeffectively utilizes the cell volume to provide a substantialimprovement in energy density, while at the time providing for improvedease of assembly and reliability of the cell module.

The present invention provides a more compact cell module by includingresilient carbonaceous means for retaining certain components of thedevice in a substantially fixed relationship with one another, suchresilient means also permitting movement of the components resultingfrom thermal expansion. Broadly, the device comprises a housingcontaining ion-conductive electrolyte means disposed between negativeand positive electrode structures and in contact therewith. Theresilient carbonaceous body preferably comprises a partially compressedfelt or cloth of carbon or graphite fibers so positioned in the deviceas to provide biasing means to maintain the cell components insubstantially fixed position during cell operation. In certainembodiments, the ion-conductive electrolyte means also serves asseparator means between the positive and negative electrode structuresor assemblies, and may also include spacer means for maintaining desiredseparation between cell components. In other embodiments of theinvention, particularly where the ion-conductive electrolyte meanscomprises a molten salt electrolyte, the separator means constitutes aseparate member disposed between the electrode assemblies or forms partof either electrode assembly, serving also to retain particulateelectrode material within the electrode assembly.

In a preferred embodiment of the invention, the device comprises ahousing, an electrolyte contained within the housing, and negative andpositive electrodes in contact with the electrolyte. The positiveelectrode comprises a body of active material confined in amaterial-holding assembly which includes at least oneelectrolyte-pervious separator member having first and secondsubstantially parallel planar surfaces, i.e., opposite faces. The firstface of the separator member is in restraining contact with the body ofactive material, and the second face faces the negative electrode. Theseparator member preferably has a porosity of from about 10 to 90percent and a median pore size of from 5 to 500 microns for retainingthe active material in place while permitting the free passage of ionstherethrough. The device further includes a spacer member abutting thesecond face of the separator member and a resilient inert body of asolid carbonaceous material, most suitably consisting of at leastpartially compressed porous cloths or felts of carbon or graphitefibrous materials having an apparent density of from about 3 to 50%,preferably 3 to 15 %, when the body is free from compression. Thisresilient carbon body maintains the spacer member and separator memberin contact with one another to retain said separator member in saidmaterial-holding assembly and maintains the positive electrode assemblyin position in the cell.

It has been discovered that a body of resilient carbonaceous material,such as woven or non-woven fibers of carbon or graphite, having a lowapparent density when said body is unrestrained is a particularlyeffective biasing means at temperatures in excess of 200°C. Suchgraphite and carbon fiber materials are commercially available and formthe subject of U.S. Pat. No. 3,107,152. Thus, the use of such aresilient carbon body in the high temperature electrical energy storagedevice of the present invention uniquely provides a means forresiliently urging certain components of the device into contact withone another while accommodating variations in the thermal expansion ofthe components when the device is charged and discharged, and heated toits various operating temperatures, which temperatures may be within therange of from 200° to 500°C or higher. Thereby, the need for bulky cellconstruction elements, such as ceramic pins and the associated structurefor retaining the separator in place, are eliminated, making feasible acell arrangement providing for greater utilization of the cell volume bythe active electrode components and, consequently, a resultant higherenergy density (watt hours/volume; and watt hours/weight) for the cell.

The use of a resilient carbon body composed of at least partiallycompressed porous woven or non-woven carbon of graphite fibrous materialis particularly advantageous in the practice of the present inventionbecause of the many benefits obtained thereby. Thus, while metals suchas Inconel-X, Hastelloy-X and other nickel and stainless steel alloyshave been suggested for high temperature use, and could be fabricatedinto spring-like members, these are considered as generally unsuitablewith respect to the other requirements present for the cell module.Thus, the solid carbonaceous resilient body is particularlyadvantageous, compared with metallic springs or resilient ceramicmaterials, in retaining its physical properties under conditions of celloperation, not being subject to creep, corrosion, or fracture for longlife operation at elevated temperatures, including temperature cycling,being readily available at low cost, and providing great ease offabrication and assembly. Accordingly, the present resilient carbonmember offers a practical and convenient solution for providing animproved high-temperature compact modular cell not heretofore known.

The resilient carbon body is essentially inert to the molten saltelectrolyte at the elevated temperatures of cell operation. However,direct contact with active material of the negative electrode, such as alithium anode, should be avoided because of corrosive interactiontherebetween.

The ion-conductive electrolyte means used in the electrical energystorage device of the present invention provides means for the transferof ions between positive and negative electrodes, i.e., constitutes anelectrolyte; provides a fixed minimum space between the electrodes; andprevents the migration of elemental active material from one electrodeto the other.

The ion-conductive electrolyte means may comprise a solid electrolytewhich will perform all three required functions of ion-transfer,spacing, and migration prevention. Examples of suitable solidelectrolytes for use in various electrochemical systems are disclosed inU.S. Pat. Nos. 3,404,035 and 3,404,036; these patents are incorporatedherein by reference. The solid electrolytes are described generally asconsisting essentially of ions of aluminum and oxygen in crystal latticecombinations and cations in an alkali metal corresponding to the activematerial (alkali metal) of the anode. When solid electrolyte is used andlocated between the positive and negative electrodes, the resilient bodyof carbonaceous material may be disposed so as to maintain theelectrodes and electrolyte in contact with one another.

Alternatively, the ion-conductive electrolyte means may comprise anelectronically non-conductive porous body wetted with a molten saltelectrolyte. In this embodiment the molten salt provides the means fortransfer of ions between the electrodes, the porous body maintaining afixed minimum space between the electrodes and also preventing theactive material of one electrode from reaching the other.

The porous body is selected from materials resistant to elevatedtemperature and which are not chemically attacked by the electrolyte oractive materials of the electrodes such as lithium. In this embodimentthe resilient body of carbonaceous material is positioned so as tomaintain the electrodes and electrolyte-wetted body in contact with oneanother.

In the presently preferred embodiment of the device, the ion-conductiveelectrolyte means comprises a molten salt electrolyte, spacer membersand separator members, the latter of which is included as a part of thepositive electrode assembly.

In a preferred embodiment, the device comprises a lithium-containingnegative electrode, a sulfur-containing positive electrode, and alithium-ion-containing molten salt electrolyte, i.e., molten at theoperating temperature of the cell or battery. In such a preferredembodiment, the lithium anode may be either a porous substrateimpregnated with liquid lithium or a solid electrode comprising alithium alloy. Examples of suitable lithium alloys include lithium-lead,lithium-tin, lithium-zinc, lithium-aluminum and combinations thereof. Aparticularly preferred negative electrode is one containing an alloy oflithium and silicon, as reported in the Application of San-Cheng Laientitled "Lithium Electrode and an Electrical Energy Storage DeviceContaining the Same", Ser. No. 512,635 filed on or about of even dateherewith and assigned to the Assignee of the present invention. Such analloy has substantially improved electrical potential and energy storagecapacity as compared with the prior art lithium alloys.

The active cathode material contained in the positive electrode assemblypreferably is one providing sulfur ions. The active cathode material maybe either elemental sulfur or a metal sulfide in particulate form.Examples of metal sulfides contemplated include the sulfides of copper,iron, tungsten, chromium, molybdenum, nickel, cobalt, and tantalum. Thesulfides of iron and copper or combinations thereof are particularlypreferred for use with a molten salt electrolyte.

The term "molten salt electrolyte" as used herein refers to a lithiumhalide-containing salt which is maintained at a temperature at or aboveits melting point during operation of the electrical energy storagedevice. The molten salt may be either a single lithium halide, a mixtureof lithium halides, or a eutectic mixture of one or more lithium halidesand other alkali metal or alkaline earth metal halides.

Typical examples of binary eutectic salts are lithium chloride-potassiumchloride, lithium chloride-magnesium chloride, lithium chloride-boronchloride, lithium bromide-potassium bromide, lithium fluoride-rubidiumfluoride, lithium iodide-potassium iodide and mixtures thereof. Twopreferred binary salt eutectic mixtures are those of lithium chlorideand potassium chloride (melting point 351°C), and lithium bromide andrubidium bromide (melting point 278°C).

Examples of ternary eutectics useful as the molten salt electrodeinclude calcium chloride-lithium chloride-potassium chloride, lithiumchloride-potassium chloride-sodium chloride, calcium chloride-lithiumchloride-sodium chloride, and lithium bromide-sodium bromide-lithiumchloride. Preferred ternary eutectic mixtures include those containinglithium-chloride, lithium fluoride and lithium iodide (melting point341°C) and lithium chloride, lithium iodide and potassium iodide(melting point 260°C).

The suitable alkali or alkaline earth metal ion should have a depositionpotential very close to or preferably exceeding the deposition potentialof the lithium ion in the electrolyte. Lithium halide salts can bereadily combined with halides of potassium, barrium, and strontium.While halides of metals such as cesium, rubidium, calcium, or sodium maybe used, these are generally less desirable for use since a substantialproportion of these metals may be codeposited with the lithium when theelectrode is charged, with some resulting small loss in potential.

Although the ternary eutectic salt mixtures, particularly thosecontaining the iodides, provide lower melting points, the binaryeutectic mixture of lithium chloride-potassium chloride sometimes ispreferred on the basis of its lower cost and availability, particularlyfor batteries to be used in large-scale applications such aselectric-powered vehicles and electric utility bulk energy storage.

In a preferred single module structural embodiment of the invention, twopositive electrode assemblies are positioned at opposite ends of thecell housing and resting on the bottom cell member of the housing. AU-shaped spacer member is in abutting contact with the facing separatormembers of the positive electrode assemblies, with the negativeelectrode assembly being a unitary two-faced structure disposed betweenthe positive electrode assemblies. A resilient body of partiallycompressed porous carbon or graphite fibrous material is packed withinthe housing, preferably at only one end, this resilient inert bodyurging the spacer member and the two separator members into contact withone another so as to maintain the positive electrode assemblies in afixed position in the cell, yet flexibly movable with respect to changesin cell temperature. Conveniently, the negative electrode assembly issuspended from the top cover of the housing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view in elevation of a compact modular cell builtin accordance with the present invention;

FIG. 2 is a sectional top plan view of the modular cell taken along thelines 2--2 of FIG. 1;

FIG. 3 is a sectional view in elevation of a simplified embodiment ofthe modular cell;

FIG. 4 is a sectional view in elevation of an embodiment of theinvention wherein the battery device includes two modular cell units;

FIG. 5 is a sectional view in elevation of another embodiment of atwo-cell device; and

FIG. 6 is a sectional top plan view taken along the lines 6--6 of FIG.5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides an electrical energy storage device whichincludes resilient means for maintaining certain components of thedevice in a substantially fixed desired relationship with one another.The design, construction, and features of the electrical energy storagedevice of the present invention will be more fully understood byreference to the attached drawings which depict certain exemplaryembodiments of the invention.

Referring now to FIGS. 1 and 2 of the drawings, thereon is shown apreferred modular single cell form of the electrical energy storagedevice of the present invention. The cell depicted therein isparticularly adaptable for use in a battery containing a plurality ofsimilar such cells each of which are electrically connected in series toprovide a higher desired battery potential, in parallel to provide adesired ampere-hour capacity, and combinations thereof. The cell 10comprises a housing 12 which contains an electrolyte 13, a negativeelectrode assembly 14, and a positive electrode assembly 16. Each of theelectrode assemblies are substantially immersed in the electrolyte.

The positive electrode assembly 16 includes a material-holding member 17which defines a cavity for receiving a body of particulate activecathode material 18. In this particular embodiment the material-holdingmember 17 is a body of dense graphite. The active material 18 isretained in place by a porous separator plate 20. Separator plate 20 issuitably and preferably formed from an electronically non-conductivematerial such as, for example, a ceramic. Examples of suitable ceramicmaterials include the oxides of aluminum, zirconium, tantalum, titanium,beryllium, boron, and combinations thereof. Advantageously, the selectedceramic should have a high degree of purity, i.e., be substantially freeof any impurities. The selected separator material may have a porosityof from 10 to 90 percent, that is to say the material may have anapparent density of from 90 to 10 percent of that of the base material.A particularly preferred porosity range is from about 20 to 40 percent.The pore size should be sufficiently small to prevent the escape of theparticulate active material 18, while still permitting the free passagetherethrough of electrolyte ions. The separator member should thereforehave a median pore size with the range of from 5 to 500 microns, andpreferably from 20 to 100 microns for use with the preferred molten saltlithium battery. The two positive electrode assemblies 16 each areprovided with electrical conductor leads 22 which are in electricalcommunication with one another.

The negative electrode assembly 14 comprises a body 24 of a lithiummaterial and is provide with an electrical conductor lead 26. The body24 of lithium material may be either a lithium alloy, as hereinbeforedescribed, or a porous substrate impregnated with lithium, or lithiumalloyed with another material to enhance the wetting of the substrate.

Intermediate the two positive electrode assemblies 16 and abuttingporous separators 20 is a spacer member 28. The spacer member depictedin the drawing is shown as a unitary U-shaped member. Obviously,however, the spacer member could comprise a plurality of adjoiningdiscrete parts, for example, two vertical side portions and a bottomportion. The material for the spacer member preferably also is aceramic. Examples of particularly preferred ceramic materials arelithium aluminate, yttrium oxide, boron nitride, silicon nitride andberyllium oxide.

Also supplied in the housing 16 is a resilient carbon body 32,preferably of woven partially compressed graphite fibers, forresiliently urging said spacer member and separator members into contactwith one another so that the active material 18 is retained in thematerial-holding member. The body of woven graphite fibers has anapparent density, when said body is unrestrained, of from 3 to 50% andpreferably from 3 to 15%. The term "partially compressed" means thatsaid body in its unrestrained state occupies a volume in excess of thatwhich is provided when present in the electrical energy storage device.The woven graphite fibers preferably have a diameter within a range offrom about 10 to 150 microns, and particularly from 20 to 75 microns.Advantageously, the body of porous woven graphite fibers is compressedto from 25 to 50 percent and preferably from 30 to 40 percent of itsunrestrained volume when it is placed in the electrical energy storagedevice. It will be appreciated that the exact amount of compression willvary depending upon, among other things, (a) the amount of force that itis desired to exert against the components, (b) upon the apparentdensity of the selected material, and (c) the number of such bodies ofwoven or nonwoven carbon and graphite fibers used. The terms "carbon"and "graphite" as used herein are essentially interchangeable for thepurposes of the present invention, and refer to cloths, felts, yarns,and tapes, both woven and nonwoven, although a resilient body of porouswoven fibers of graphite is generally preferred for its superiorphysical properties.

In the preferred embodiment depicted in FIGS. 1 and 2, the activecomponents of electrical energy device 10 (active lithium body, activecathode material, and electrolyte) are electronically isolated fromhousing 12 by a plurality of insulator members 30.

Advantageously, the active components of the cell also are isolated fromthe atmosphere, for example, by a cover member 34, which encloses and isin sealing engagement with housing 12. Cover member 34 is retained byfastening means such as a plurality of threaded fasteners 40. The covermember also includes a plurality of apertures 36, each of which areprovided with seals 38 through which extend electrical conductor leads22 and 26. In FIG. 1 the two conductor leads 22 of the positiveelectrode assembly are shown connected together external to the cover.It generally is preferred, however, that the interconnection of theelectrode assemblies be internal to reduce the number of openingsrequired in the cover member, particularly when a number of cells areinterconnected to form a battery having a desired voltage andampere-hour capacity.

In a preferred embodiment of the device, the electrolyte is a salt whichis molten at the operating temperature of the battery. For suchembodiment, some means must be provided to heat the device to itsoperating temperature. Heat may be provided by an external source suchas by placing the device in a furnace or other enclosure. In theembodiment depicted in FIGS. 1 and 2 the heating means comprises anouter liner 42 containing a body of thermal insulating material 44, anda plurality of heating elements 46, for example, electrical resistanceheaters.

Referring now to FIG. 3, a simplified embodiment is depicted of anelectrical energy storage device in accordance with the invention. Thedevice comprises a housing 12, an electrolyte 13, a negative electrodeassembly 14 and a single positive electrode assembly 16. The devicefurther includes a spacer member 28 abutting housing 12 and porousseparator member 20 of positive electrode assembly 16. A body 32 of atleast partially compressed woven graphite fibers is located betweenpositive electrode assembly 16 and the adjacent end wall 48 of housing12 for resiliently urging positive electrode assembly 16, includingseparator member 20, into contact with spacer member 28, whereby thecell components are maintained in a substantially fixed desiredrelationship.

Referring now to FIG. 4, a two-cell electrical energy device is depictedutilizing the modular concept shown in FIGS. 1 and 2. The devicedepicted in FIG. 4 includes a housing 12 containing a plurality oftwo-faced negative electrode assemblies 14 and a plurality of positiveelectrode assemblies 16. The negative electrode assemblies 14 andpositive electrode assemblies 16 are arranged such that each negativeelectrode assembly is located intermediate a pair of positive electrodeassemblies. Each positive electrode assembly 16 has a porous separatormember 20 in facing relationship with a negative electrode assembly,each face comprising active electrode material. Intermediate porousseparator members 20 of each pair of positive electrode assemblies 16 isa spacer member 28. Located intermediate the two middle positiveelectrode assemblies 16 is a partially compressed body of woven graphitefibers 32 for resiliently urging the positive electrode assemblies,separator members, and spacer members into contact with one anotherthereby maintaining the assemblies and members in a substantially fixeddesired relationship. It will be appreciated that the precise locationof the body of partially compressed woven graphite fibers is notcritical. For example, whereas it is shown in FIG. 4 as intermediate thetwo positive electrode assemblies, it will be obvious that resilientcarbon body 32 could be placed at either or both ends of the electricalenergy storage device.

Referring now to FIGS. 5 and 6, therein is dipicted yet anotherembodiment of the electrical energy storage device of the presentinvention. In this particular embodiment the positive electrodeassemblies each comprise a plurality of electronically non-conductiveelements which form a housing for confining the active material of thepositive electrode assembly. Specifically, the apparatus 100 includes ahousing 112 for containing an electrolyte 113. In contact with theelectrolyte contained therein are a plurality of negative and positiveelectrode asemblies. The negative electrode assemblies 114 may be any ofthose hereinbefore described. The positive electrode assemblies includetwo end electrodes 116 and an intermediate electrode 116a. The endelectrodes 116 each include a material-holding member for containing abody of active cathode material 118. Each of these material-holdingmembers include a bottom member 152, side members 148, a back member150, and a porous separator front member 120. Members 148, 150, 152, and120 are preferably all formed from the electronically nonconductiveceramics described hereinbefore. Only the member in facing relationshipto a negative electrode assembly or anode need be a porous ceramicseparator member; the remaining members of electrodes 116 may be dense,substantially liquid-impervious materials to provide electricalinsulation between housing 112 and the active elements of the device(active cathode material, anode, and electrolyte). Since intermediateelectrode 116a is located between two negative electrode assemblies 114,its material-holding member comprises two porous separator members 120forming the front and back walls, with two side members 148 and bottommember 152 located therebetween and defining a cavity for confining abody of active cathode material 118. Located between positive electrodeassembly 116a and each end positive electrode assembly 116 is a spacermember 128. Intermediate either or both, as shown, of each end positiveelectrode assembly 116 and housing 112 is a partially compressed body ofwoven graphite fibers 132 for maintaining said members and assemblies ina predetermined substantially fixed relationship with one another duringoperation of the battery.

The advantageous features of the electrical energy storage device of thepresent invention is thus readily apparent to those versed in the art.Specifically, since a resilient carbonaceous means now is available formaintaining a plurality of components in a substantially fixedrelationship, it is possible to utilize components having substantiallyplanar parallel surfaces, which components may be closely spacedtogether thus obtaining a high utilization of the available volumetricspace of any given device. Indeed, in accordance with the presentinvention, a 50%, savings in space requirements is obtainable comparedwith the techniques heretofore utilized for high-temperature electricalenergy storage devices such as lithium/molten salt/metal sulfidebatteries. Accordingly, a substantial increase in the energy density ofthe cell is obtained.

The following non-limiting example is set forth as illustrative of theadvantageous features provided by the device of the present invention.

EXAMPLE

An electrical energy storage device substantially the same as thatdepicted in FIGS. 1 and 2 was constructed. The electrical energy storagedevice included a housing 12 of stainless steel containing a molten saltelectrolyte 13 consisting of a eutectic mixture of LiCl-KCl (m.p.352°C), a negative electrode (anode) assembly 14, and two positiveelectrode (cathode) assemblies 16. The anode assembly 14 was a porousmetal substrate impregnated with liquid lithium. Cathode assemblies 16each included a dense graphite material-holder member 17 having a cavityfor containing a body of active material 18 consisting of particulateiron sulfide. The cavity of containing the body of active material wascovered with a flat porous separator member 20 having substantiallyparallel planar faces, a porosity of about 40% and a median pore sizewithin the range of from 20 to 100 microns. One of the porous separatormembers 20 consisted of an oxide of aluminum, and the material used forthe other porous separator member was an oxide of magnesium. Spacermembere 28 located intermediate the two cathode assemblies 16 wasU-shaped and formed from three pieces of a dense oxide of beryllium.Insulator members 30 were formed from a dense, impervious oxide ofaluminum. Located between one cathode assembly 16 and insulator member30 was an inert resilient body 32 consisting of a felt of partiallycompressed carbon fibers having an apparent density of about 4% when inan unrestrained or uncompressed state. The felt body of carbon fiberswas compressed to about 50% of its free volume.

The electrical energy storage device was connected to a source of powerand a load. The device was heated to its operating temperature (about400°C) and then was cycled by alternately discharging and charging it ata predetermined constant current and for a preselected period of time.The device was operated for about 60 cycles over a period of 61 days atan average coulombic efficiency of 96%; during which period about 100%of the theoretical energy storage capacity was utilized indicating noloss of the active cathode material 18 from either cathode assembly 16.During operation the temperature varied from a low of 380° to a high of450°C, thus clearly demonstrating the advantageous feature of thepresent invention for maintaining certain of the components in a desiredsubstantially fixed relationship with one another at elevatedtemperatures over a varying temperature range. Upon disassembly noevidence of cracks or breakage of any of the components as a result ofexcess stress was observed.

When the partially compressed woven carbon fibers 32 are removed andreplaced with a high-temperature alloy metallic biasing means, e.g.,high-temperature wrought stainless steels and Cr-Co-Mo alloys, as, forexample, in the form of a metallic spring, or as a body of compressedmetallic fibers, the biasing means gradually loses its resiliency whenthe device is maintained at its operating temperature, i.e., about400°C. Such loss in resiliency results in separation of the componentsand loss of the active cathode material 18 to the molten saltelectrolyte. Obviously, the loss of active material to the electrolytealso results in a substantial loss in battery capacity.

When it is attempted to maintain the various members of the electricalenergy storage device in a desired relationship utilizing a fixed rigidmeans, such as either ceramic pins or a nonresilient wedge, generallyoccupying a substantially greater volume of the cell, various members ofthe electrical energy device are often found to crack or break uponheating of the device to its operating temperature. Thus, the electricalenergy device of the present invention is clearly advantageous overthese prior art techniques.

While the foregoing invention has been described with reference tocertain preferred embodiments, and is particularly advantageous for usein a molten salt electrolyte cell operating at elevated temperatures,numerous variations will be apparent to those versed in the art. Forexample, in place of the molten salt electrolyte used in the modularcell, a solid state electrolyte or an organic electrolyte also can beutilized. Further, while the preferred active cathode materials havebeen illustrated and described as metal sulfides, it will be equallyapparent to those versed in the art that the oxides and halides ofvarious metals also could be utilized. In addition, while the resilientcarbonaceous body described herein is particularly advantageous andeffective when consisting of at least partially compressed carbon orgraphite fibers, other more dense or solid resilient carbonaceous bodiesmay be used, less desirably, for certain specialized applications.However, such dense carbon bodies shaped in various conventionalspring-like configurations are considerably bulkier and less stablestructurally than the preferred fibrous forms. It should further benoted that the resilient carbonaceous bodies may be disposed at variousother locations in the cell structure to provide the desired maintenanceof cell components in relatively fixed position. It is not intended,therefore, that the invention be limited to the exemplary embodimentsdescribed and illustrated, but rather its scope is to be determined withreference to the objects thereof taken together with the followingappended claims.

What is claimed is:
 1. An electrical energy storage device comprising:a.a housing having side and end walls and a bottom member forming acontainer; b. a molten salt electrolyte contained within said housing;c. a negative electrode assembly in contact with said electrolyte; d. apositive electrode assembly in contact with said electrolyte and spacedfrom said negative electrode assembly and comprising a body of activematerial retained within a material-holding member; e. anelectrolyte-permeable separator member having first and second opposingsurfaces disposed between said negative and positive electrodeassemblies, said first surface contacting said body of active material,said second surface facing and spaced from said negative electrodeassembly, said porous separator member retaining said active material inplace while permitting the free passage of ions therethrough; f. aspacer member abutting said second surface of said separator member; andg. a resilient body of at least partially compressed carbon or graphitefibers contained within said housing adjacent said positive electrodeassembly for resiliently urging said spacer and separator members intocontact with one another to maintain at least said members insubstantially fixed position.
 2. The device of claim 1 wherein saidresilient carbonaceous body, when free from compression, has an apparentdensity of from about 3 to 50 percent.
 3. The device of claim 1 whereinsaid material-holding member includes said porous separator member. 4.The device of claim 1 wherein said material-holding member comprises abody of dense graphite having opposite surfaces and an open cavity inone of said surfaces for receiving said body of active material.
 5. Thedevice of claim 1 wherein said material-holding member comprises aceramic body having a cavity therein for receiving said body of activematerial.
 6. The device of claim 1 wherein there is provided a pluralityof positive and negative electrode assemblies, and each of saidplurality of negative electrode assemblies is located intermediate twopositive electrode assemblies.
 7. The device of claim 1 wherein saidnegative electrode assembly has first and second substantially parallelplanar surfaces, and wherein there is provided two positive electrodes,one in spaced relation to the first surface and the other in spacedrelation to the second surface of said negative electrode assembly. 8.The device of claim 7 wherein each positive electrode includes a porousseparator member and wherein a spacer member is located intermediatesaid positive electrodes and abuts the separator member of each of saidpositive electrodes.
 9. A high-temperature, compact modular electricalenergy storage device comprising:a housing containing device componentsincluding metal sulfide positive and lithium negative electrode meanspositioned therein, ion-conductive electrolyte means disposed betweensaid positive and negative electrode means and in contact therewith,said electrolyte comprising a lithium halide salt which is molten at theoperating temperature of the device, and a resilient carbonaceous bodyof at least partially compressed carbon or graphite fibers locatedadjacent the positive electrode means for resiliently urging at leastselected device components in contact with one another and maintainingthem in substantially fixed position during operation of the device atelevated temperatures.
 10. The device of claim 9 wherein said resilientcarbonaceous body, when free from compression, has an apparent densityof from about 3 to 50 percent.
 11. The device of claim 9 wherein saidresilient carbonaceous body is disposed between a wall of the housingand the positive electrode.
 12. The device of claim 9 wherein saidion-conductive electrolyte means comprises porous separator meanscontaining said electrolyte within its pores, and said resilientcarbonaceous body is disposed between a wall of the housing and thepositive electrode.
 13. The device of claim 12 wherein a porousseparator is maintained in contact with the positive electrode.
 14. Ahigh-temperature compact modular electrical energy storage devicecomprising:a. a housing having side and end walls and a bottom memberforming a container; b. an electrolyte contained within said housing,said electrolyte comprising a lithium salt which is molten at theoperating temperature of the device; c. a lithium negative electrodeassembly in contact with said electrolyte; d. a positive electrodeassembly in contact with said electrolyte and spaced from said negativeelectrode assembly and comprising a body of particulate metal sulfideretained within a material-holding member; e. an electrolyte-permeableseparator member having first and second opposing surfaces disposedbetween said negative and positive electrode assemblies, said firstsurface contacting said body of particulate metal sulfide, said secondsurface facing and spaced from said negative electrode assembly, saidporous separator member having a median pore size of from 5 to 500microns for retaining said body of particulate metal sulfide in placewhile permitting the free passage of ions therethrough; f. a spacermember abutting said second surface of said separator member, and g. aresilient body of at least partially compressed carbon or graphitefibers contained within said housing, said resilient body having anapparent density, when free from compression, of from about 3 to 50percent for resiliently urging said spacer and separator members intocontact with one another to maintain at least said members insubstantially fixed position.
 15. The device of claim 14 wherein saidmaterial-holding member of said positive electrode comprises side wallsspaced from one another, top and bottom members spaced from one anotherand in contact with said side walls forming a container having two openends, and two end members, one located at each of said open ends formingan enclosed cavity for retaining said body of particulate metal sulfide,at least one of said end members constituting said porous separatormember.
 16. The device of claim 15 wherein there is provided a pluralityof positive and negative electrode assemblies, and each of saidplurality of negative electrode assemblies is located intermediate twopositive electrode assemblies.